This investigation is designed to continue the studies of the intrarenal distribution of blood flow and of renin secretion carried out in primary and essential hypertension over the last five years. The Xenon 133 method for estimation of renal blood flow is used in studying renal blood flow in man. Microsphere studies have been introduced for the measurement of glomerular blood flow distribution in the rat and a parallel study of normotensive and hypertensive rats has begun to further elucidate the findings of a relationship between renin secretion and renal blood flow distribution in hypertensive man. Renin activity is measured in man by the radioimmunoassay and in the animal by a microradioimmunoassay method. Studies of renal blood flow in the rat under a wide variety of stimuli including intraarterial infusion of epinephrine, norepinephrine, acetylcholine, phenoxybenzamine have been carried out in conjunction with the studies of renal blood flow in man under a variety of alterations in sodium balance. These studies are being continued in an effort to elucidate the role of the kidney and the effects of renal blood flow and renin secretion in the causation or continuation of hypertension in man and to determine the possible role of the kidney in this disease.